Universal Serial Bus

USB (Universal Serial Bus) is a widely-used standard for connecting, communicating with, and supplying power to devices.

Originally developed in the mid-1990s, USB has become the primary interface for connecting peripherals such as keyboards, mice, printers, storage devices, and more to computers and other digital systems.

• Plug and play: Devices connected via USB are typically recognized automatically by the operating system, making setup easy without needing complex configurations.
• Hot-Swappable: USB devices can be connected and disconnected without shutting down the host computer.
• Power Delivery: USB cables can provide power to devices, often eliminating the need for separate power adapters. This is especially beneficial for portable devices.

Operation

USB (Universal Serial Bus) operates on a polling mechanism to communicate with connected devices, rather than relying on devices to initiate communication.

Host-Centric Communication

USB uses a host-controller model, where the host (typically a computer or a main controller in embedded systems) manages all communication.
USB devices (e.g., keyboard, printer, storage device) are peripherals that cannot initiate data transfer by themselves. They rely on the host to request data or send commands.

USB Data Transfer Types

Depending on the type of data a device transmits, USB uses different transfer types, each with specific polling intervals:

Control Transfers

Used for device setup and configuration, these transfers are used to establish and maintain communication between the host and device, and it is usually short and high priority

Interrupt Transfers

Used for devices requiring low-latency response, such as keyboards and mice.
The host polls interrupt transfer devices at regular intervals to detect any changes (like a key press).

Bulk Transfers

Used for large data transfers, often found in storage devices like USB flash drives.
Bulk transfers are polled less frequently than interrupt transfers and are handled when the bus is less busy.

Isochronous Transfers

Used for real-time data, like audio or video streams, where timing is critical, but occasional data loss is acceptable.
The host sets a fixed data rate for polling isochronous devices, but it doesn’t wait for confirmation from the device, as real-time data streaming requires constant flow.

Communication Workflow

1. Device Connection and Enumeration:

When a USB device is connected, the host detects it and assigns it a unique address.
The host then queries the device’s descriptors, which provide details like device type, manufacturer, power needs, and supported data transfer types.

2. Data Transfer Based on Polling

The host controller schedules polling based on the device type and transfer type. For instance, a keyboard (using interrupt transfers) will be polled very frequently, while a flash drive (using bulk transfers) may be polled only as needed.
If the device has data to send, it responds with data packets. If not, it simply indicates it has no data.

3. Power and Bandwidth Management

USB manages power and bandwidth allocation across devices, optimizing the communication to prevent data loss or delays.
USB hubs can further extend this polling, distributing polling signals and power to devices in a hub-and-spoke architecture.

Versions

USB 1.0/1.1

Basic speeds for peripherals like keyboards and mice, Up to 12 Mbps (Megabits per second)

USB 2.0

Improved speeds, allowing connections with higher data needs, like external hard drives, Up to 480 Mbps

USB 3.0/3.1/3.2

Increased data transfer rates, suitable for high-speed devices, Up to 5 Gbps (Gigabits per second), 10 Gbps, and 20 Gbps, respectively

USB4

Builds upon Thunderbolt 3, supporting up to 40 Gbps and improving power delivery and data throughput, Up to 40 Gbps

USB Version	Communication Type	Mode	Description
USB 1.0 / 1.1	Serial	Half-Duplex	Basic USB standard; data flows in one direction at a time.
USB 2.0	Serial	Half-Duplex	Improved speed, but data still flows in only one direction at a time.
USB 3.0 / 3.1 / 3.2	Serial	Full-Duplex	Allows simultaneous two-way communication, increasing data rate and efficiency.
USB4	Serial	Full-Duplex	Uses Thunderbolt 3 technology; supports higher data rates and advanced features like DisplayPort.

Connector Type

There are several types of USB connectors, each suited to specific purposes or devices:

USB-A

The standard rectangular port found on most computers.

USB-B

Typically used for larger devices like printers.

Mini-USB

Older mobile devices and small electronics.

Micro-USB

Widely used for mobile devices before USB-C.

USB-C

A newer, reversible connector found on most modern devices, supporting higher speeds and power delivery.

Wiring Diagrams

wiring diagrams for different USB versions and types, showing the evolution of wire configurations from USB 1.0 to USB-C with USB4.

USB 1.0 / USB 2.0 (Standard-A Connector)

Type: USB-A
Version: USB 1.0 / 2.0
Wires: 4 (VCC, D-, D+, GND)

  USB 1.0 / 2.0 Wiring Diagram

  +----------+----------+----------+----------+
  |  Red     |  White   |  Green   |  Black   |
  |  (VCC)   |   (D-)   |   (D+)   |  (GND)   |
  +----------+----------+----------+----------+

• VCC (Red): Supplies 5V power to the device.
• D- (White) and D+ (Green): Differential data lines for half-duplex communication.

• GND (Black): Ground wire.

  USB 3.0 / USB 3.1 (Standard-A Connector)

  Type: USB-A
  Version: USB 3.0 / 3.1
  Wires: 9 (VCC, D-, D+, GND, SSRX+, SSRX-, SSTX+, SSTX-, GND)

  USB 3.0 / 3.1 Wiring Diagram
  
  +-----------+------------+------------+-------------+
  | Red/(VCC) | White/(D-) | Green/(D+) | Black/(GND) |
  +-----------+------------+------------+-------------+
  +--------------+----------------+
  | Blue/(SSRX-) | Yellow/(SSRX+) |
  +--------------+----------------+
  +----------------+----------------+-------------+
  | Purple/(SSTX-) | Orange (SSTX+) | Black/(GND) |
  +----------------+----------------+-------------+

• SSRX- (Blue) and SSRX+ (Yellow): SuperSpeed receive data lines.
• SSTX- (Purple) and SSTX+ (Orange): SuperSpeed transmit data lines.
• Additional GND (Black): An additional ground pin to support the SuperSpeed lanes.
  The D- and D+ lines maintain compatibility with USB 2.0 devices, while the SuperSpeed lanes enable full-duplex data transfer.

USB-C (Supports USB 3.2 and USB4)

Type: USB-C
Version: USB 3.2 / USB4
Wires: 12+ (VCC, GND, D-, D+, SSRX+, SSRX-, SSTX+, SSTX-, CC1, CC2, SBU1, SBU2)

  USB-C Wiring Diagram

  +----------+----------+----------+----------+----------+----------+----------+----------+
  |  VCC     |  D-      |  D+      |  GND     |  SSTX+   |  SSTX-   |  SSRX+   |  SSRX-   |
  +----------+----------+----------+----------+----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |  CC1     |  CC2     |  SBU1    |  SBU2    |
  +----------+----------+----------+----------+

• SSTX+ / SSTX- and SSRX+ / SSRX-: SuperSpeed transmit and receive lanes for full-duplex communication.
• CC1 and CC2: Configuration channels for detecting cable orientation, power negotiation, and USB mode selection.
• SBU1 and SBU2: Sideband use channels, used for alternate modes like video output (e.g., DisplayPort).
  The USB-C connector supports USB4 with additional capabilities for higher data rates (up to 40 Gbps), video output, and power delivery up to 100W.